1. Technical Field
The present invention relates generally to semiconductor fabrication, and more particularly, to a ground shield for a semiconductor device and a related method.
2. Related Art
In silicon radio frequency (RF) applications, magnetic fields created by current flowing through RF passive elements (e.g., inductors, capacitors, transmission lines, etc.) creates eddy currents in the underlying conductors and silicon substrate. As a result, the magnetic field dissipates power in the silicon, and degrades the quality factor of the RF passive element. To address this situation, the lossy silicon substrates are often shielded from RF passive elements by a metal ground shield or ground plane, which provides a low resistance return path for the magnetic field-induced electrical current. A ground shield is typically located on top of the silicon substrate and under the RF passive element to block eddy currents from entering the substrate.
Current practice is to use the available first metal (M1) wiring level to provide a ground shield in various complementary metal-oxide semiconductors (CMOS), bipolar CMOS (BiCMOS), silicon germanium (SiGe) BiCMOS, RF-CMOS, etc., technologies. This approach, however, suffers from a number of drawbacks. For example, ideally the ground shield should be as thick as possible to minimize the resistance and block as much of the eddy currents as possible. In contrast, the first metal wiring level is ideally as thin as possible to facilitate tight pitch wiring. For example, a typical 90 nm CMOS generation first metal (M1) wiring pitch and height are approximately 220 nm and approximately 240 nm, respectively; while a passive inductor pitch and height are approximately 2.4 μm and approximately 3 μm, respectively. Accordingly, if the ground shield is made thicker, the aspect ratio of the metal wire is increased, which detrimentally results in higher metal resistances for the ground shield. The ground shield also disadvantageously increases capacitance between conductors and ground. This situation occurs, for example, in aluminum (Al) or copper (Cu) metal systems. For a low loss ground shield, the wire resistance should be very negligible, i.e., the ground shield needs to act as a nearly ideal ground with minimal power loss. For power amplifier technologies, in particular, the return loss on the ground shield can be a substantial limiting factor in efficiency (i.e., wasted power in inductors due to I*I*R*f).
In view of the foregoing, there is a need in the art for an improved ground shield.